Levamisole, marketed as the hydrochloridesalt under the trade name Ergamisol (R12564), is an anthelmintic and immunomodulator belonging to a class of synthetic imidazothiazole derivatives. It was discovered in 1966 at Belgium's Janssen Pharmaceutica, where it was prepared initially in the form of its racemate called tetramisole.[1] The two stereoisomers of tetramisole were subsequently synthesized, and the levorotatory isomer was given the name levamisole.[2] Levamisole was originally marketed in humans to treat parasitic worm infections. However, Levamisole was withdrawn from the U.S. and Canadian markets in 1999 and 2003, respectively, due to the risk of serious side effects and the availability of more effective replacement medications.[3][4] The most serious side effect of levamisole is agranulocytosis, a severe depletion of white blood cells that leaves patients vulnerable to infection. More recently, Levamisole has been studied in combination with other forms of chemotherapy for the treatment of colon cancer, melanoma, and head and neck cancer. Currently, Levamisole remains in veterinary use as a dewormer for livestock.

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Levamisole was originally used as an anthelmintic to treat worm infestations in both humans and animals. Levamisole works as a nicotinic acetylcholine receptor agonist that causes continued stimulation of the parasitic worm muscles, leading to paralysis. In countries that still permit the use of levamisole, the recommended dose for anthelmintic therapy is a single dose, with a repeated dose 7 days later if needed for a severe hookworm infection.[8] Most current commercial preparations are intended for veterinary use as a dewormer in cattle, pigs, and sheep. However, levamisole has also recently gained prominence among aquarists as an effective treatment for Camallanusroundworm infestations in freshwatertropical fish.[9]

After being pulled from the market in the U.S. and Canada in 1999 and 2003, respectively, levamisole has been tested in combination with fluorouracil to treat colon cancer. Evidence from clinical trials support its addition to fluorouracil therapy to benefit patients with colon cancer. In some of the leukemic cell line studies, both levamisole and tetramisole showed similar effect.[10]

An interesting adverse side effect these reviewers reported in passing was "neurologic excitement". Later papers, from the Janssen group and others, indicate levamisole and its enantiomer, dexamisole, have some mood-elevating or antidepressant properties, although this was never a marketed use of the drug.[12][13]

One of the more serious side effects of Levamisole is agranulocytosis, or the depletion of the white blood cells. In particular, neutrophils appear to be affected the most. This occurs in 0.08-5% of the studied populations.[14] There have also been reports of levamisole induced necrosis syndrome in which erythematous painful papules can appear almost anywhere on skin.

Levamisole is readily absorbed from the gastrointestinal tract and metabolized in the liver. Its time to peak plasma concentration is 1.5–2 hours. The plasma elimination half-life is fairly quick at 3–4 hours which can contribute to not detecting Levamisole intoxication. The metabolite half-life is 16 hours. Levamisole's excretion is primarily through the kidneys, with about 70% being excreted over 3 days. Only about 5% is excreted as unchanged Levamisole.[15][16]

Drug testing of racehorse urine has led to the revelation that among Levamisole equine metabolites are both pemoline and aminorex, stimulants that are forbidden by racing authorities.[17][18][19] Further testing confirmed aminorex in human and canine urine, meaning that both humans and dogs also metabolize levamisole into aminorex.[20] The possible stimulant properties of aminorex are what has caused Levamisole to be used as a cocaine adulterant which is described in more detail below.

Levamisole may be quantified in blood, plasma, or urine as a diagnostic tool in clinical poisoning situations or to aid in the medicolegal investigation of suspicious deaths involving adulterated street drugs. About 3% of an oral dose is eliminated unchanged in the 24-hour urine of humans. A post mortem blood levamisole concentration of 2.2 mg/l was present in a woman who died of a cocaine overdose.[21][22]

Levamisole has increasingly been used as a cutting agent in cocaine sold around the globe with the highest incidence being in the U.S.A . In 2008–2009, levamisole was found in 69% of cocaine samples seized by the Drug Enforcement Administration (DEA).[6] By April 2011, the DEA reported the adulterant was found in 82% of seizures.[23]

Levamisole adds bulk and weight to powdered cocaine (whereas other adulterants produce smaller "rocks" of cocaine) and makes the drug appear purer.[24] In a series of investigative articles for The Stranger, Brendan Kiley details other rationales for levamisole's rise as an adulterant: possible stimulant effects, a similar appearance to cocaine, and an ability to pass street purity tests.[25]

Levamisole suppresses the production of white blood cells, resulting in neutropenia and agranulocytosis. With the increasing use of levamisole as an adulterant, a number of these complications have been reported among cocaine users.[6][26][27] Levamisole has also been linked to a risk of vasculitis,[28] and two cases of vasculitic skin necrosis have been reported in users of cocaine adulterated with levamisole.[29]

Levamisole-tainted cocaine was linked to several high-profile deaths. Toxicology reports showed levamisole, along with cocaine, was present in DJ AM's body at the time of his death.[30] Andrew Koppel, son of newsman Ted Koppel, was also found with levamisole in his body after his death was ruled a drug overdose.[31] More recently it has also been suspected in the death of a Sydney teenager.[32]

In response to the dangers, The Stranger, People's Harm-Reduction Alliance and DanceSafe began producing tests to identify levamisole's presence in cocaine. The kits include a survey postcard, and one revealed its presence in a 1/4-kg block of cocaine, indicating both users and dealers were using the kits.[33]

The original synthesis at Janssen Pharmaceutica resulted in the preparation of a racemic mixture of two enantiomers, whose hydrochloride salt was reported to have a melting point of 264–265 °C; the free base of the racemate has a melting point of 87–89 °C. When the two enantiomers were made separately, the levorotatory (S-(−)-) enantiomer, subsequently called levamisole, was found to have a melting point of 227–229 °C as its hydrochloride salt, and 60–61.5 °C as the free base. Thus, 60 °C is entered as the value for melting point in the info box. The dextrorotatory (R-(+)-) enantiomer, subsequently called dexamisole, has a melting point of 227–227.5 °C as its hydrochloride salt, and 60–61.5 °C as the free base.[34]

It is used to immobilize the nematode C. elegans on glass slides for imaging and dissection.[37]

In a C. elegans behavioral assay, analyzing the time course of paralysis provides information about the neuromuscular junction. Levamisole acts as an acetylcholine receptor agonist, which leads to muscle contraction. Continuing activation leads to paralysis. The time course of paralysis provides information about the acetylcholine receptors on the muscle. For example, mutants with fewer acetylcholine receptors may paralyze slower than wild type.[38]

^J. Scarth et al. (2010) "The use of in vitro drug metabolism studies to complement, reduce and refine in vivo administrations in medication and doping control." Proceedings of the 18th International Conference of Racing Analyists and Veterinarians. pp 213-222